Digital projection systems have become substantially more popular, more powerful and less expensive in recent years. Digital projection systems generally include a spatial light modulator for producing an image, and a light source to illuminate the light modulator and associated optical elements to project the image onto a projection surface. There are a variety of types of digital projection systems that have been developed and gained widespread use in recent years. These include liquid crystal display (LCD) systems (both reflective and transmissive), liquid crystal on silicon (LCOS) displays, and digital mirror devices (DMD), among others.
To provide adequate projection illumination, digital projection systems often employ an arc lamp as the light source. Arc lamps generally include a pair of electrodes having a gap between them. A high voltage electric arc is caused to jump across this gap, and in so doing produces the desired light. One type of arc lamp that is frequently used is a Xenon arc lamp. Xenon arc lamps allow “instant-on” functionality (unlike ultra-high pressure (UHP) lamps that have a slower turn-on time) and provide a good color gamut because the xenon emission spectrum is relatively high and flat in the visible range. Xenon lamps are also more environmentally friendly because, unlike UHP lamps, they do not contain mercury.
In systems employing an arc lamp, including projection systems and other devices, the lifetime and brightness of the lamp is a significant factor for convenience, economy, and user satisfaction. If the lamp has a short lifetime, the user will be required to replace it often, which is both inconvenient and expensive. Additionally, if the brightness of the lamp diminishes too much during its lifetime or fluctuates noticeably over relatively short time periods, this can greatly diminish the satisfaction of the user.
There are a number of factors that can contribute to a decrease in the output of an arc lamp over time. One of these factors is variation in the gap between electrodes in the lamp. Variation in the electrode gap in an arc lamp has a great effect on the quality and brightness of the lamp. When the electrode gap is comparatively small, the fireball or plasma ball produced between the electrodes is likewise small and relatively compact. However, when the gap is larger, the plasma ball will tend to be more stretched out and diffuse. This phenomenon is of particular interest because the light of the arc is usually reflected by an elliptical or otherwise curved reflector. When the arc fireball is larger, the image of it that is reflected by the reflector will be larger, and thus less light will be coupled into the subsequent optical systems. In projection systems, the integrator tunnel/homogenizer that receives the light from the lamp is typically of industry standard dimensions. For maximum light coupling (and thus maximum light projection efficiency), it is desirable for the plasma to be as compact as possible, and as close as possible to the focal point of the reflector.
There are several factors that tend to change the electrode gap in arc lamps. One of these is long-term electrode burn back. As an arc lamp is used over its lifetime, the electrode tips will tend to burn and ablate away, thus reducing the length of the electrodes and thereby increasing the electrode gap. The long-term widening of the electrode gap due to electrode burn back produces an ever-larger fireball, causing the brightness of the lamp to gradually diminish over the lifetime of the lamp as described above.
Another factor that affects the electrode gap is short-term thermal expansion. As a lamp heats up from the time the arc is initially struck until it reaches a state of thermal equilibrium, the size of the electrode gap and the center of its position can vary due to the coefficients of thermal expansion of the anode cathode, reflector, and other lamp components. Since the various components of the lamp are made of different materials having differing coefficients of thermal expansion, the electrode gap when the device is not in use and at room temperature will normally vary from the electrode gap when the device is operating and at thermal equilibrium. Differential thermal expansion of the lamp and light engine parts can cause the fireball image and other parts of the device to move relative to each other, thus reducing coupled light.